1. Field of the Invention
The invention relates to a method and apparatus for the electrolytic coating of one side of a moving metal strip.
2. Description of the Prior Art
EP-A-1257O7 describes an electrolyte coating method in which the moving metal strip as cathode is in contact with an electrically conductive outer surface of a rotating cathode roller and an insoluble anode is positioned concentrically with the roller over a part of the circumference of the roller at a distance from the strip. A slot is thus formed over that circumference part into which electrolyte is fed and in which the coating takes place, the electrolyte flowing generally through the gap at an average velocity such that turbulent flow occurs. The electrolyte is fed as a fluid jet into the gap at one of its ends with a tangential component relative to the path of the strip. This method of electrolyte coating strip has a number of advantages compared with other known methods.
EP-A-28298O discloses a similar apparatus, in which the electrolyte is fed in at the strip exit end of the slot.
Where the current is fed to the strip via the roller, it does not need to be led with resistance losses along the strip, as is the case with flat, vertical or horizontal cells, but rather it may be transferred directly from the cathode roller to the strip; this advantage is of particular importance for thin strips such as for example when plating tinplate with a thickness of for example 0.17 mm. A second advantage is that (in contrast with flat, vertical or horizontal cells where the strip is led between two anodes positioned at a distance from the strip) the path of the strip is fixed, because the strip is taken around the cathode roller. This means that the gap between the strip and the anode varies less during coating, especially if the anode is an insoluble one, thereby achieving a more uniform thickness of the coating layer.
In spite of the above mentioned advantages it has been found from experiments carried out by the applicant on the method of EP-A-125707 that it has a number of disadvantages. First of all the uniformity of the thickness of the coating layer is not satisfactory across the width of the strip. Secondly, under certain conditions the efficiency of the known method may be very low especially at somewhat higher strip speeds. These disadvantages will be further illustrated below.